Explanation:

- Option 4: Leon Festinger

- Correct answer. Leon Festinger introduced the concept of cognitive dissonance in 1957. It details the discomfort experienced when holding conflicting thoughts or beliefs.

- Option 1: Erikson

- Known for his theory on psychosocial development, detailing eight stages of human development, not cognitive dissonance.

- Option 2: J. Piaget

- Famous for his theory of cognitive development in children, focusing on how children acquire and process knowledge.

- Option 3: G. H. Mead

- Contributed to social psychology, especially concerning the development of the self and societal roles, but not cognitive dissonance.